Plugging control for electric motors



Oct. 9, 1934. P. D. CROUT PLUGGING CONTROL FOR ELECTRIC MOTORS v Filed Aug. 20,1930

Patented Oct; 9, i934 TES ENT QFFIQ PLUGGING CONTROL FOR ELECTRIC MOTORS Prescott D. Crout, Schenectady, N. Y., assignor to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application August 20, 1930, Serial No. 476,504

3 Claims.

The present invention relates to control of electric and other motors by plugging and has for its object to provide improved means for stopping a motor and terminating the power supply when it has come substantially to a standstill and before it begins to rotate in a reverse direction.

The accompanying drawing illustrates two systems embodying the invention.

Figure 1 shows the invention applied to an alternating current motor, wherein the motor is disconnected from the line when the speed of its shaft approaches zero value, while 1 Fig. 2 shows the invention as applied to a slip ring type motor, whereby the motor is disconnected when the frequency in the rotor of the motor approaches that obtaining at standstill.

Referring to Fig. 1, a relay 3. has a magnet frame 2 equipped with shunting pole pieces 3. Wound on one side of the magnet frame is a coil 4 which is connected in series with a second coil 5. The coil 5, together with a third coil 6, is mounted on an armature 7 of the electromagnet. This armature is pivotally supported on the side of the magnet frame 2 opposite to that on which coil 4 is wound and it carries at its upper end a movable contact 8a, which is adapted to engage stationary contacts 81) at either side of the armature. A three phase motor Q'has connected to its driving shaft a rotating brush 10, which engages intermittently stationary contacts 11 whenever the motor revolves. The brush 10 completes intermittently a circuit when passing over the sta tionary segments 11. An electromagnetic switch 12 having a winding 12a and normally open contacts 12b and 120 serves to connect the motor to the lines L and L for normal operation, while a second electromagnetic switch 13 having a magnet coil 13a and normally open contacts 131), 13c, serves to connect the motor to the lines for plugging. Switch 13 has in addition normally open auxiliary contacts 13d and 136. The circuits of the magnetswitches are controlled by a double throw switch 14 having stationary contacts 14a and 14b and a movable contact or a knife blade 140. There is also provided a relay 15 having a magnetic winding 15a, normally closed contacts 15b and normally open contacts 150.

Referring more particularly to relay 1, the coils 4, 5 and 6 should be designed so that their reactance is small relative to their resistance in order that the current flowing in thecoils is substantially in phase with their terminal voltage. It will be understood that the coil 4 produces a flux, part of which passes entirely through the iron of the frame 2, while another portion thereof passes through the shunt poles 3 and thus affects the armature 'i. On the other hand, the coils 5 and 6 produce fluxes which, after passing through the armature 7, bridge the air gap between the latter and the pole pieces and then travel through the pole pieces and through the iron frame. If the fluxes produced by the coils 4, 5 and 6 are substantially in phase with each other, they will exert a pull on the armature 7,

which is normally biased to the center position, in

one directtion or the other, depending upon the relative polarity of the coils l, 5 and 6 respectively.

The coil 5 is normally wound with a relatively small number of turns, while the coil 6 is wound with a larger number of turns, so that with the coil 5 energized, the pull due to the action of coils 4 and 5 upon the armature is a given direction, whereas with the coil 6 energized the flux initially induced in the Cel er 301%, the coil 5, is reversed so that th ull on the armature is opposite to the is di ction. Thus, by alternately opening and clo c rcuit of the coil 6, the pull on the arms. periodically reversed, the frequency or re sal depending upon the frequency of the int ruptions. This characteristic of the relay is made use f present invention as will be more later.

The controller operates in the following man ner:

The lines L L and L furnish 3 phase alter nating current, the line L being directly con nected to one terminal of-the'motor. When it is desired to operate the mo'torunder normal conditions, the switch blade 140 is moved to complete circuit with the lower contact 1411. This connects coil 12a of switch 12 across lines L and L thereby closing the contacts 12b and 120 and completing the connection of the motor to the lines L and L The motor therefore starts and comes up to speed in the usual manner. If it is now desired to stop the motor by plugging, the switch 14 is moved upward, completing circuit with the stationary contacts 145. This deenergizes the switch 12, thus disconnecting the motor from lines L and L At the same time, circuit is completed from L through coil 13c, normally closed contacts 15b of relay l5 and contacts 14b and 14c to line L Thus the switch 13 is energized, thereby closing its contacts and reconnecting the motor 9 to the lines L and L but in the reverse direction. The motor thereupon acts as a generator, supplying power back to the line and thus slows down.

4 and 5, and contacts 1311, to line L ly at a frequency depending upon the speed of the motor and thus the armature '7, due to the reversal of the pull described heretofore, tends to oscillate from one side to the other. As long as the speed of the motor, and thus the frequency of the interruptions of the circuit from coil 6, is high, the amplitude of the vibration of the armature 7 is not sufljlcient to cause the contacts 8a to engage the contacts 8b. As the motor slows down, however, the frequency of interruptions of the circuits of the coil 6 decreases, and thus the frequency of the oscillations of the armature also decreases, urtil ultimately the motor approaches standstill. The frequency of the reversals of the armature is then sufficiently low, so that its amplitude increases to complete circuit between the contacts 811 and one or the other stationary contactsjBb. This, completes a circuit from line L through contacts 8a and 81) through coil 15a, contactsl ib and 140 to line L thus energizing relay 15 and-opening normally closed contacts 15b and closing normally open contacts 150. The opening of contacts 15b de-energizes the switch 13 and thus disconnects the motor from the line, while the closing of contacts 150 maintains the circuit of the coil 15a independently of the contacts 8a and 8b. The relay '15- thus remains energized and prevents the switch 13 from reclosing until switch 14 is opened, which restores the equipment to the initial starting position.

The system shown in .Fig. 2 employs a relay similar to that employedin the system of Fig. 1 but the relay is represented only by its coils 20 and 21 corresponding respectively to the coils 4 and 5 of Fig. 1 and its contacts 36 corresponding to contacts 8a in Fig: 1. An induction motor 22 of the slipring type has primary terminals 23, 24 and 25 and secondary-terminals 26, 27 and 28, the latter being connected through a 3 phase variable resistance 29. A double throw reversing switch 30 is connected to the lines 15 and L and its center poles are connected to the normally closedcontacts 33 and 34 of a magnet switch 31, having a winding 32 and a normally open contact 35. The coil 20 of the relay is connected to the primary terminals 23 and 24 of the motor, while the coil 21 is connected to the secondary terminals 27 and 28. It will be observed that when the motor runs at full speed, the frequency in the secondary circuit of the motor and" therefore in the coil 21 is very low, so that the difference in frequency in the two coils of the relay is very great. Thus the direction of the pull on the armature of the relay varies rapidly and the armature cannot oscillate far enough to close the contacts 36. However, as.

the motor approaches standstill the frequency in its secondary circuit and\hence the frequency. in

the coil 21 approaches that of the coil 20, so that the frequency of oscillations of armature of the I Assume that the reversing switch 30 is thrown upward and the motor runs at full speed in a given direction. If it is now desired to stop the motor by supplying reverse power thereto or plugging it, the reversing switch is thrown in the downward direction, thereby reversing the connection between the lines L and L and the terminals 23 and 24respectively. Reversal of the power causes themotor to slow down and to approach standstill. When the motor approaches zero speed, the frequency in the coil 21 approaches that of the coil 20 and the relay closes the contacts 36 in the aforedescribed manner, thereby energizing coil 32 of the switch 31 and opening normally closed contacts 33 and 34 thus disconnecting motor terminals 23 and 24 from the line and terminating power supply to the motor. At the same time the normally open contact 35 closes, thus maintaining switch 31 energized independently of the contacts 36 and preventing re-establishment of power supply to the motor until reversing switch 30 is opened when switch 31 is again de-energized and normal conditions are restored.

It is to be understood that this invention is not limited to the specific type of relay shown in the drawing, as it is obvious that any type of relay which responds to the difference of two frequenciesin the manner described may be employed.

Also, the constant reference frequency as well as the variabl'e frequency which responds to the motor speed may be generated in any other well known manner than that described.

What I claim as new and desire to secure by Letters Patent is:

1. In a system for stopping electric motors, in

combination, plugging means and automatic control means to limit duration of action of the former means, said automatic means comprising a relay having an armature and means to produce a unidirectional torque upon said armature and further means to reverse said torque with a frequency variable with the motor speed.

2. In combination a motor, a control relay therefor, having an armature, means to produce a unidirectional torque upon said armature and means to reverse said torque with a frequency which is a function of the motor speed.

3. In a system for stopping a motor by plugging, in combination, a switch for reversing the power supply to said motor, and a relay for controlling saidswitch upon reverse power connection, said relay having an armature, a coil energized from 

